Traditionally, in the production of semiconductor integrated circuits and displays, a pattern as defined by a mask is exposed onto an area ("die") of a sensitive substrate (termed herein generally a "wafer"). Normally, multiple such exposures are made at each die on the wafer, each exposure being made using a different mask. Such multiple exposures place extreme demands on the ability of the apparatus to align the die and mask very accurately.
Alignment prior to exposure is normally achieved by scanning or otherwise irradiating an electron beam on a mark formed on the mask and/or on the surface of the wafer. Electrons in the beam (i.e., "primary electrons") that are reflected, scattered, and/or produced as secondary electrons ("SEs") propagating from the mark are detected to determine whether the mark(s) are properly aligned. For example, marks on the mask are conventionally detected by detecting electrons reflected from certain features of the mark when the electron beam strikes the mark. Marks as used on wafers normally comprise grooves each having walls and a bottom. When the electron beam strikes the mark, primary electrons that have penetrated into the wafer are scattered within the thickness dimension of the wafer. Scattered electrons that pass through the walls of the grooves provide strong detection signals. Electrons reflected from the bottoms of the grooves provide relatively weak detection signals since the trajectory of such electrons is impeded by the groove walls. Such signal differences are measured and analyzed to determine the relative position of the mark.
Mark detection by reflected electrons as described above tends to produce signals having low-contrast and poor resolution. First, the detector used in such schemes can detect only reflected electrons that propagate to an area within the field of view of the detector, which tends to be limited. Second, the electrons that reflect from the mark normally represent no more than about 50% of the primary electrons impinging upon the mark. Third, if the mark is relatively thin or otherwise has an elevation that is not much different from the surrounding surface, irradiating the mark with an electron beam having a high accelerating voltage results in little interaction between the primary electrons and the mark.
Scanning-type imaging apparatus such as scanning electron microscopes (SEMs) do not exploit reflected primary electrons as described above, but rather produce an image of a region on a specimen by detecting SEs generated by the interaction of irradiating primary electrons impinging upon the region. SEM techniques have been used for detecting alignment marks and other features (e.g., contact holes) on a wafer. For example, an electron beam having a high accelerating voltage is directed at the bottom of a contact hole. Electrons are reflected from the bottom of the hole and propagate upward at an angle through a resist layer on the surface of the wafer. SEs produced when the reflected electrons pass through the surface of the resist are detected. (See Japanese Laid-Open Patent Document no. 5-290786 (1993)).
Conventional image production by detecting SEs also poses problems. First, the primary-electron beam must be directed diagonally (i.e., at a substantial angle of incidence) at the specimen, which cannot be accomplished in lithographic apparatus in which the electron beam normally has a zero incidence angle relative to the mask and substrate. Second, an electrical field generated by the secondary-electron (SE) detector to attract SEs to the detector can adversely affect impingement of the primary electrons on the mark. These problems degrade detection accuracy and cause a decreased signal-to-noise (S/N) ratio. Another problem is that SEs around the optical axis generate beam instability caused by charging, as discussed in Kato et al., Jpn. J. Appl. Phys. 35:6429-6434 (1996).